This invention relates to an improved method and apparatus for heating a thermoplastic article to a temperature at which it begins to soften so that it may be subsequently manipulated by a technician into a desired shape, and, more particularly, it is directed to the shaping of thermoplastic eyeglass frames.
By way of background, in the past many types of devices were used to heat eyeglass frames by conduction, convection and radiant heat. However, all of the foregoing had certain deficiencies. In those devices which utilized conduction to transfer heat to the frames from heated glass beads or salts, undesirable dimples and residue were formed on the frames. In commercial devices which used convection, such as disclosed in my U.S. Pat. No. 2,789,200, issued Apr. 16, 1957, the heat transfer by heated forced air took an undesirably long time. Prior devices which used radiant heat produced by high temperature radiators were very difficult to use and control because there were many variables which had to be considered including (1 ) the color of the thermoplastic being treated which affected the absorption of energy from the radiator, and (2 ) the critical positioning of the article relative to the radiator. To overcome the difficulties attendant to the use of radiant energy, my U.S. Pat. No. 3,816,705, issued June 11, 1974, was developed using far infrared to permit effective control of radiant infrared radiation relative to the article being treated. Furthermore, in all of the above prior types of devices heating means such as electrical resistance elements were used to heat something else, such as the air in convection heaters, or the cast aluminum as used in radiation heaters. This type of indirect heating produced an inertial heat lag and thus there was an undesirable time delay before the devices were capable of producing the required temperature. It is with overcoming the foregoing deficiencies of prior types of thermoplastic heating devices that the present invention is concerned.
By way of further background, infrared radiation travels at the speed of light in straight lines out from a source, and this radiation can be controlled and directed by optical systems, reflectors and prisms in the same manner as visible light. The wave length of the radiation varies inversely with the temperature of the radiation source, that is, the higher the temperature of the radiation source, the shorter will be the wave length, and vice versa. Furthermore, many materials are transparent to the shorter wave lengths. Hence, the heating of an article with the longer rays which are effective in heating the surface and with the shorter rays which are effective in penetrating into the interior of the article will decrease the amount of time necessary to bring such article up to an elevated temperature. In addition, if the article is also exposed to a flow of hot air, the heating time will be still further reduced. By cycling tungsten halogen lamps, or other radiation generators, on and off the generation of energy output can be varied and can be closely controlled. In addition, the cycling produces both long wave and shortwave radiation because, as noted above, the wave length varies with the temperature of the source.
It is an important object of the present invention to provide a heater which not only heats thermoplastic articles to bending temperature rapidly, but which can also be brought to operating temperature almost instantaneously, and which can produce a closely controlled temperature.
It is another important object of the present invention to provide a heater for thermoplastic articles with the capability of heating a thermoplastic article with both long and short wavelength infrared rays. A related object is to effect the foregoing and also simultaneously provide a bath of heated air to the article to thereby uniformly bring the article to the desired temperature rapidly.
Another important object of the present invention is to use as a heat generator for heating thermoplastic articles a tungsten halogen lamp or its equivalent, which has a very small inertial heat lag. A related object is to subject the tungsten lamp to a pulsating electric current varying from approximately zero voltage to maximum at intervals to cause said lamp to radiate infrared rays in both the long and short wave lengths to produce near and far infrared, to thereby produce rapid and thorough heating of the article.
Another object of the present invention is to control the energy received by the article being treated by cycling a radiant energy emitting lamp on and off to vary the energy output therefrom.
A further object of the present invention is to utilize a portion of radiation from the heating source to actuate an interrupter control which produces the intermittent pulsations to thereby regulate the energy output. A related object is to use the interrupter in an air cooling system and vary its exposure to infrared radiation from the heat generator while simultaneously varying its exposure to the cooling air so as to control and vary the on and off periods of the interrupter and thus further regulate energy output.
Yet another object of the present invention is to provide an unique energy guide system for directing the radiation from a source to opposite sides of the article being treated in such a manner that there are no unequal concentrations of radiation which might deleteriously affect the article.
A further object of the present invention is to provide a device for heating thermoplastic articles which includes a thermal converter surrounding the source of radiant energy whereby the radiant energy is transformed into heat, whereby air flowing around the generator and through the converter may be heated and then directed to the article to be heated.
A still further object of the present invention is to provide an improved heat responsive switch assembly capable of responding rapidly to radiant heat and which is also capable of providing different lengths of cycles of operation by varying the amount of exposure to radiant energy which the switch receives.